1. Field of the Invention
The present invention relates to a cellular system, a mobile station, a base station, and a transmission power control method as well as a program to be executed for implementing the method, and more particularly to a method of controlling a transmission power for a high speed data transmission from a base station through a down-link to a mobile station in a cellular system.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
In 3rd Generation Partnership Project, a high speed downlink packet access (HSDPA) has been proposed as a method for a high speed data transmission through a down-link from a base station to a mobile station in the cellular system.
This high speed down-link packet access (HSDPA) utilizes a high-speed physical down-link shared channel (HS-PDSCH) for data transmission through the down-link from the base station to the mobile station. This high-speed physical downlink shared channel (HS-PDSCH) is used for data transmission from each base station to a plurality of mobile stations, wherein the base station or its control station decides a schedule for time division data transmissions at different timing to the plural mobile stations.
Each dedicated physical channel (DPCH) is provided between each base station and each of plural mobile stations belonging to the each base station for bi-directional data transmissions between the each base station and the plural mobile stations belonging to the each base station, wherein the each dedicated physical channel (DPCH) provides a down-link for transmitting signals or data from the base station to the mobile station and a up-link for transmitting the signals or data from the mobile stations to the base station.
The each mobile station has a small ratio of time to receive data through the high-speed physical downlink shared channel (HS-PDSCH). Notwithstanding, the each mobile station is configured to continuously set the dedicated physical channel (DPCH) to the base station, to which this mobile station belongs, during a stand-by state for receiving data, so as to enable a prompt commence of the data transmission upon receipt of a request for the data transmission. The each base station can establish a current data transmission to one of the plural mobile stations, while can establish plural the dedicated physical channels (DPCH) to the remaining plural mobile stations which are in the stand-by state.
A soft handover can be used in the cellular system for concurrently establishing plural channels between each mobile station and plural base stations. Plural base stations transmit a common pilot signal at a predetermined transmission power, so that each of the mobile stations can establish the dedicated physical channel (DPCH) to one of the plural base stations, which has transmitted the common pilot signal with a maximum received power. Namely, the each mobile station receives the plural common pilot signal with possible different receiving powers from the plural base stations. The each mobile station detects one common pilot signal with the maximum receiving power, so that the each mobile station establishes the dedicated physical channel (DPCH) to the base station which has transmitted the common pilot signal with the maximum receiving power. In accordance with the soft handover technique, the each mobile station further establishes another dedicated physical channel (DPCH) to another base station which has transmitted the common pilot signal with a receiving power which is slightly lower than the above maximum receiving power. Namely, the each mobile station can establish concurrently the plural dedicated physical channels (DPCH) to plural base stations. The base station which establishes the dedicated physical channel (DPCH) during the soft handover will, hereinafter, be referred to as “link base station”.
The cellular system utilizes a high speed closed loop transmission power control. For control of the transmission power through the up-link of the dedicated physical channel (DPCH), the base station utilizes a discrete pilot signal included in the up-link signal transmitted through the up-link in order to measure a received signal-to-interference-ratio (SIR) and compare the measured value to a predetermined target signal-to-interference-ratio (SIR). If the measured value is smaller than the target value, then the base station generates a transmit power control bit (TPC_UP) which indicates the power-up, and transmits the down-link signal including this transmit power control bit (TPC_UP) to the mobile station through the down-link. If the measured value is not smaller than the target value, then the base station generates another transmit power control bit (TPC_DOWN) which indicates the power-down, and transmits the down-link signal including this transmit power control bit (TPC_DOWN) to the mobile station through the down-link. Upon receipt of the transmit power control bit (TPC), the mobile station adjusts to increase or decrease the transmission power based on the received transmit power control bit (TPC).
A method of deciding the transmission power control from the received transmit power control bit (TPC) is disclosed in 3GPP TS25. 214 v.5.1.0 (2002-06) “3rd Generation Partnership Project:Technical Specification Group Ratio Access Network:Physical Layer Procedures (FDD) (Release 5)”. In this method, the following two algorithms are presented.
In accordance with the first algorithm, upon receipts every time of the transmit power control signals (TPC), the mobile station adjusts the transmission power of the dedicated physical channel (DPCH) in accordance with the received transmit power control signal (TPC). In case that the transmit power control signals (TPC) are used together with the soft handover, the mobile station receives respective transmit power control signals (TPC) from the plural link base stations. If at least one of the received plural transmit power control signals (TPC) is TPC_DOWN, then the mobile station decreases the transmission power of the dedicated physical channel (DPCH). If all the received plural transmit power control signals (TPC) are TPC_UP, then the mobile station increases the transmission power of the dedicated physical channel (DPCH). This transmission power control enables that a receiving quality of the up-link signal satisfies the target signal-to-interference-ratio (SIR), and also prevents that receiving qualities of all the up-link signals to all the link base stations exceed the target signal-to-interference-ratio (SIR), as an upper limit, thereby avoiding any substantive increase of an interference wave power of the up-link.
In accordance with the second algorithm, the mobile station does not change or adjust the transmission power until the mobile station has received the transmit power control signals (TPC) five times after the mobile station has made the last transmission power control. Upon the fifth receipt of the transmit power control signal (TPC) after the mobile station has made the last transmission power control, then the mobile station adjusts the transmission power based on a synthesized signal of the five received transmit power control signals (TPC). If all of the five received transmit power control signals (TPC) indicate the increase of the transmission power, then the synthesized transmit power control signal (TPC) also indicates the increase of the transmission power. If all of the five received transmit power control signals (TPC) indicate the decrease of the transmission power, then the synthesized transmit power control signal (TPC) also indicates the decrease of the transmission power. If all of the five received transmit power control signals (TPC) do not indicate either the decrease or the increase uniformly, then the synthesized transmit power control signal (TPC) indicates no change of the transmission power.
In case that the last-described second algorithm is used together with the soft handover, the mobile station does not change or adjust the transmission power until the mobile station has received the transmit power control signals (TPC) five times from each of the link base stations after the mobile station has made the last transmission power control to each of the link base stations. Upon the fifth receipt of the transmit power control signal (TPC) from each of the link base stations after the mobile station has made the last transmission power control to the each of the link base stations, then the mobile station adjusts the transmission power to the each of the link base stations based on the each synthesized signal of the five received transmit power control signals (TPC) from the each of the link base stations. Namely, the mobile station generates respective synthesized signals for the plural link base stations, respectively, wherein each of the synthesized signals depends on the five received transmit power control signals (TPC) from each of the plural link base stations. If the number of the synthesized signals indicating the decrease of the transmission power exceeds a predetermined number, then the mobile station decreases the transmission power to the plural link base stations. If the number of the synthesized signals indicating the increase of the transmission power exceeds a predetermined number, then the mobile station increases the transmission power to the plural link base stations. If the number of the synthesized signals indicating the decrease of the transmission power exceeds the predetermined number and the number of the synthesized signals indicating the increase of the transmission power exceeds the predetermined number, then the mobile station does not change the transmission power to the plural link base stations.
Consequently, the use of the second algorithm makes the transmission power control cycle slower as compared to the use of the first algorithm. The slow transmission power control cycle means an undesired slow follow-up to the variation in quality of the received signals. The synthesis of the five or plural transmit power control signals (TPC) reduces the probability of undesired erroneous controls to be made. The co-use of the second algorithm with the soft handover allows the mobile station to increase the transmission power, but only when the number of the link base stations requesting the increase of the transmission power exceeds the predetermined number. This is effective to reduce the interference wave power of the up-link.
As described above, the transmission power control and the soft handover are effective techniques for reducing the transmission power and the interference wave power in order to increase a line capacity in a wire-less access system, particularly a code division multiple access (CDMA).
FIG. 1 is a schematic view illustrative of the general operation in the conventional cellular system. During soft handover, as shown in FIG. 1, a mobile station 32 can concurrently establish plural dedicated physical channels (DPCH) to plural base stations 31-1 and 31-2, while the mobile station 32 can concurrently establish only a single high-speed physical downlink shared channel (HS-PDSCH) to one base station 31-1. Since the high-speed physical downlink shared channel (HS-PDSCH) is the common channel, then it is necessary to notice which mobile station is set as a destination for receiving a transmission packet.
Informations about the mobile station as the destination for receiving the transmission packet and also control informations necessary for receiving the packet are transmitted through a high-speed shared control channel (HS-SCCH). It should also be noted that the mobile station 32 can concurrently establish only a single high-speed shared control channel (HS-SCCH) to one base station 31-1. Further, it is necessary that the mobile station 32 sends the base station 31-1 a delivery acknowledge notification (acknowledge/non-acknowledge signal: ACK/NACK signal) which indicates whether or not the mobile station 32 has safely received the packet transmitted through the high-speed physical downlink shared channel (HS-PDSCH). This delivery acknowledge notification is transmitted through a high-speed dedicated physical control channel (HS-DPCCH) as the up-link from the mobile station 32 to the base station 31-1.
FIG. 2 is a diagram illustrative of respective formats of the high-speed dedicated physical control channel (HS-DPCCH) as the up-link and the dedicated physical channel (DPCH) as the other up-link. The dedicated physical channel (DPCH) comprises a dedicated physical control channel (DPCCH) and a dedicated physical data channel (DPDCH). The dedicated physical control channel (DPCCH) includes discrete pilot channels “Pilot”, discrete transmit power control bits “TPC”, and discrete feedback informations “FBI”. The dedicated physical data channel (DPDCH) comprise data “Data” which include user informations and control informations. The dedicated physical control channel (DPCCH) and the dedicated physical data channel (DPDCH) are modified and multiplexed for subsequent transmission.
The high-speed dedicated physical control channel (HS-DPCCH) has a single slot length corresponding to three slots of each of the dedicated physical control channel (DPCCH) and the dedicated physical data channel (DPDCH). One slot of the dedicated physical control channel (DPCCH) comprises a single set of the pilot channel “Pilot”, the transmit power control bit “TPC” and the feedback information “FBI”. One slot of the dedicated physical data channel (DPDCH) comprises a single set of data “Data”. The single slot length of the high-speed dedicated physical control channel (HS-DPCCH) includes a delivery acknowledge notification (acknowledge/non-acknowledge signal: ACK/NACK signal) and a channel quality indicator (CQI) which indicates the quality of the down-link. A judgement of the acknowledge/non-acknowledge signal (ACK/NACK) included in the high-speed dedicated physical control channel (HS-DPCCH) is made by the base station which has the high-speed dedicated physical control channel (HS-DPCCH) and has transmitted the packet. For this reason, any diversity synthesis between the base stations is not made.
The up-links of the dedicated physical channels (DPCH) are subjected to a diversity synthesis between the link base stations 31-1 and 31-2, wherein the diversity synthesis is made by a wire-less network control station (BSC) as shown in FIG. 1. This dedicated physical channel (DPCH) is subjected to the above-described high speed closed loop transmission power control, so that the dedicated physical channel (DPCH) has a predetermined receiving quality. A transmission power (PH) of the high-speed dedicated physical control channel (HS-DPCCH) is equal to a sum of a transmission power (PD) of the dedicated physical channel (DPCH) and a predetermined offset power (.).PH=PD+.  (1)
The base station, which transmitted the packet, judges whether or not the mobile station has safely received the packet, based on the received acknowledge/non-acknowledge signal (ACK/NACK) included in the high-speed dedicated physical control channel (HS-DPCCH). If the base station judged that the mobile station has not safely received the packet based on the non-acknowledge signal (NACK), then the base station will transmit again the same packet to the mobile station for avoiding any packet loss.
If the delivery acknowledge notification erroneously and incorrectly indicates “ACK” even the mobile station has in fact not safely received the packet, then the base station miss-judged that the mobile station has safely received the packet, and the base station does not transmits the packet again, whereby the mobile station does not receive the packet. Namely, the problem with the packet loss will be raised. It is necessary that a receiving error rate of the non-acknowledge signal (NACK) is much lower than another receiving error rate of the acknowledge signal (ACK). In other words, a sufficiently high receiving quality of the acknowledge/non-acknowledge signal (ACK/NACK) at the base station which transmitted the packet is needed.
If no countermeasure is made to the above-issues, then this allows a deterioration of the receiving quality of the acknowledge/non-acknowledge signal (ACK/NACK) through the following operations or processes. During the soft handover, a transmission power of an up-link dedicated physical channel (UL-DPCH) as a reference to the transmission power of the dedicated physical channel (DPCH) is controlled as follows. If the received signal-to-interference-ratio (SIR) of the up-link dedicated physical channel (UL-DPCH) is larger than a predetermined reference signal-to-interference-ratio (SIR), then the link base station transmits the transmit power control signal (TPC) which indicates the decrease of the transmission power. If the synthesized received signal-to-interference-ratio (SIR) is smaller than the predetermined reference signal-to-interference-ratio (SIR), then the link base station transmits the transmit power control signal (TPC) which indicates the increase of the transmission power.
During the soft handover, in accordance with the first algorithm, the mobile station receives the respective transmit power control signals (TPC) from the respective link base stations. If all of the received transmit power control signals (TPC) indicate the increase of the transmission power (TPC_UP), then the mobile station increases the transmission power of the dedicated physical channel (DPCH). If at least one of the received transmit power control signals (TPC) indicate the decrease of the transmission power (TPC_DOWN), then the mobile station decreases the transmission power of the dedicated physical channel (DPCH).
During the soft handover, in accordance with the second algorithm, the mobile station receives the respective transmit power control signals (TPC) from the respective link base stations. If the number of the synthesized signals indicating the decrease of the transmission power exceeds a predetermined number, then the mobile station decreases the transmission power to the plural link base stations. If the number of the synthesized signals indicating the increase of the transmission power exceeds a predetermined number, then the mobile station increases the transmission power to the plural link base stations. If the number of the synthesized signals indicating the decrease of the transmission power exceeds the predetermined number and the number of the synthesized signals indicating the increase of the transmission power exceeds the predetermined number, then the mobile station does not change the transmission power to the plural link base stations.
In the above-case, even if the received signal-to-interference-ratio (SIR) of the up-link dedicated physical channel (UL-DPCH) of the packet-transmitting base station is less than the reference signal-to-interference-ratio (SIR), and if the received signal-to-interference-ratios (SIR) of the up-link dedicated physical channels (UL-DPCH) of the other base stations are more than the reference signal-to-interference-ratio (SIR), and if the above-conditions for increasing the transmission power are not satisfied, then the mobile station does not increase the transmission power of the up-link dedicated physical channels (UL-DPCH), thereby causing a deterioration of the received signal-to-interference-ratio (SIR) of the up-link dedicated physical channel (UL-DPCH) of the packet-transmitting base station, resulting also in a deterioration in the receiving quality of the high-speed dedicated physical control channel (HS-DPCCH).
There are two conventional methods for solving the above problems. The first conventional method is that the mobile station controls the transmission power of itself based on only the transmit power control signal (TPC) transmitted from the packet-transmitting base station even when the mobile station executes the soft handover. This first conventional method will hereinafter be referred to as “first conventional technique”. In accordance with this first conventional technique, the transmission power of the mobile station is so controlled that the received signal-to-interference-ratio (SIR) of the up-link dedicated physical channel (UL-DPCH) of the packet-transmitting base station satisfies the reference signal-to-interference-ratio (SIR), in order to improve the receiving quality of the high-speed dedicated physical control channel (HS-DPCCH), thereby improving the receiving quality of the acknowledge/non-acknowledge signal (ACK/NACK).
In accordance with the first conventional technique, only when the mobile station is on the receipt of the packet from the packet-transmitting base station, then the mobile station performs the transmission power control based on only the transmit power control signal (TPC) transmitted from the packet-transmitting base station. If the mobile station is not on the receipt of the packet, then the mobile station performs the normal transmission power control during the soft handover. If the mobile station does not receive the packet and the acknowledge/non-acknowledge signal (ACK/NACK) is not transmitted for the high-speed dedicated physical control channel (HS-DPCCH), then any increase of the up-link interference wave power is prevented. This first conventional technique is disclosed in the article “R1-02-0537 entitled “Discussion on ACK/NACK Signaling” filed in the first meeting #25 of 3GPP RAN (radio Access Network) WG (Working Group).
A similar conventional technique, which will hereinafter be referred to as a second conventional technique, is disclosed in the article “R1-02-0929 entitled “Enhanced HS-DPCCH power control in soft handover” filed in the first meeting #27 of 3GPP RAN (radio Access Network) WG (Working Group).
In accordance with the second conventional technique, the transmit power control signal (TPC) transmitted from the packet-transmitting base station is compared to a synthesized signal of all the transmit power control signals (TPC) transmitted from all of the link base stations, so that the number of times of detected differences between the both signals is cyclically counted for each predetermined cycle time period “K”. As the counted number just prior to the transmission of the acknowledge/non-acknowledge signal (ACK/NACK) is large, then an off-set power to be added for commencing the transmission of the acknowledge/non-acknowledge signal (ACK/NACK) is large.
The detection of the difference between the above-both signals means that even the target signal-to-interference-ratio (SIR) is not satisfied in the packet-transmitting base station and the transmit power control signal (TPC) indicating the increase of the transmission power is transmitted from the packet-transmitting base station, while the other link base stations have higher receiving qualities than the target signal-to-interference-ratio (SIR), whereby the transmit power control signals (TPC) indicating the decrease of the transmission power are transmitted from the other link base stations. This means that a propagation loss at the packet-transmitting base station is higher than other propagation losses at the other link base stations. As the propagation loss at the packet-transmitting base station becomes high, then a probability of the difference between the both transmit power control signals (TPC) becomes also high, and the off-set power to be added becomes high, thereby improving the receiving quality of the acknowledge/non-acknowledge signal (ACK/NACK) at the packet-transmitting base station.
In accordance with the second conventional technique, after the off-set power is added and the transmission of the acknowledge/non-acknowledge signal (ACK/NACK) is commenced, then the mobile station performs the transmission power control based on only the transmit power control signal (TPC) transmitted from the packet-transmitting base station similarly to the above-described first conventional technique.
In accordance with the above-described conventional method of controlling the transmission power, it is possible in the following case that the receiving quality of the high-speed dedicated physical control channel (HS-DPCCH) during receipt of the packet is deteriorated.
In the normal transmission power control except when the mobile station receives the packet, the transmission power of the mobile station is decreased or remains unchanged, provided that at least one of the link base stations satisfies the target signal-to-interference-ratio (SIR). During receipt of the packet, even if the other link base stations satisfy the target signal-to-interference-ratio (SIR), then the transmission power control is made by giving a priority to the packet-transmitting base station, wherein the transmission power is increased until the target signal-to-interference-ratio (SIR) becomes satisfied at the packet-transmitting base station.
In general, the above-described transmission power control giving the priority to the packet-transmitting base station needs a higher transmission power than the normal transmission power control. In accordance with the first conventional technique, immediately after the mobile station is placed into a packet receiving state and the mobile station is shifted from the normal transmission power control to the transmission power control giving the priority to the packet-transmitting base station, then it is highly possible that the transmission power is not sufficiently high for satisfying the target signal-to-interference-ratio (SIR) of the up-link dedicated physical channel (UL-DPCH) of the packet-transmitting base station. If the transmission power is not sufficiently high, then it takes a long time to increase the transmission power up to a sufficiently high power level. Until the transmission power becomes sufficiently high, the receiving quality of the high-speed dedicated physical control channel (HS-DPCCH) is low.
In general, the packets through the down-links have been received and multiplexed and the presence or absence of the receiving error has been confirmed, before the acknowledge/non-acknowledge signal (ACK/NACK) is transmitted at the high-speed dedicated physical control channel (HS-DPCCH). FIG. 3 is a timing chart illustrative of the first conventional technique of the transmission power control in the cellular system. As shown in FIG. 3, a necessary control signal for receiving the packet is received at the high-speed shared control channel (HS-SCCH). This causes a predetermined time delay “TD” of a transmission time “TT” of transmitting the acknowledge/non-acknowledge signal (ACK/NACK) from a packet receiving state time “TS” at which the mobile station is placed in a packet receiving state.
Under conditions shown in FIG. 3, during this time delay “TD”, the transmission power of the mobile station is increased up to a sufficiently high transmission power level for satisfying the target signal-to-interference-ratio (SIR) at the packet-transmitting base station, whereby no substantive deterioration of the acknowledge/non-acknowledge signal (ACK/NACK) is caused.
In the following case, during the time delay “TD”, the transmission power of the mobile station is not increased up to a sufficiently high transmission power level for satisfying the target signal-to-interference-ratio (SIR) at the packet-transmitting base station, whereby a substantive deterioration of the acknowledge/non-acknowledge signal (ACK/NACK) is caused.
First, if the propagation loss at the packet-transmitting base station is much larger than the propagation losses at the other link base stations, then it is possible that the continuous increase of the transmission power during the time delay “TD” does not cause the increased transmission power to reach the target sufficient high power level.
Second, a high receiving error rate of the transmit power control signal (TPC) transmitted from the packet-transmitting base station may cause a high probability of decreasing the power by the mobile station due to the receiving error of the transmit power control signal (TPC), even the packet-transmitting base station indicates the increase of the transmission power, whereby the transmission power is not increased up to the target sufficient high power level.
Third, a delayed control cycle of the transmission power control results in a reduced umber of time of the transmission power control during the time delay “TD”, while a single time increase of the transmission power is limited in its increasing level, whereby the transmission power is not increased up to the target sufficient high power level, resulting in an increased probability of appearance of the deterioration of the acknowledge/non-acknowledge signal (ACK/NACK).
In accordance with the above-described second conventional technique, the off-set power is added for subsequent transmission of the acknowledge/non-acknowledge signal (ACK/NACK), wherein the off-set power depends on a probability that the propagation loss at the packet-transmitting base station becomes higher than the propagation losses at the other link base stations in the predetermined time period “K”. This means that the time period “K” may be selected so that the off-set power compensates the difference of the propagation loss at the packet-transmitting base station and the propagation losses at the other link base stations, even if the difference is large. This second conventional technique needs the mobile station to cyclically count the number of times of detected differences between the both signals for each predetermined cycle time period “K”. This means that the mobile station has an increased load to the process.
In the above circumstances, the development of a novel cellular system free from the above problems is desirable.